The IEEE 802.11n standard for wireless communications, expected to be finalized in late 2008, incorporates multiple-input multiple-output (MIMO) multiplexing into the orthogonal frequency-division multiplexing (OFDM) technology adopted by previous versions of the 802.11 standard. MIMO systems have the advantage of considerably enhanced throughput and/or increased reliability compared to non-multiplexed systems.
Rather than sending a single serialized data stream from a single transmitting antenna to a single receiving antenna, a MIMO system divides the data stream into multiple streams which are modulated and transmitted in parallel at the same time in the same frequency channel, for example by multiple transmitting antennas. At the receiving end, one or more MIMO receiver antenna chains receives a linear combination of the multiple transmitted data streams, determined by the multiple paths that can be taken by each separate transmission. The data streams are then processed, as described in more detail below.
In general, a MIMO system employs multiple transmit antennas and multiple receive antennas for data transmission. A MIMO channel formed by the NT transmit and NR receive antennas can support up to NS datastreams, where NS≦min{NT, NR}.
In a wireless communication system, data to be transmitted is first modulated onto a radio frequency (RF) carrier signal to generate an RF modulated signal that is more suitable for transmission over a wireless channel. In general, for a MIMO system, up to NT RF modulated signals may be generated and transmitted simultaneously from the NT transmit antennas. The transmitted RF modulated signals may reach the NR receive antennas via a number of propagation paths in the wireless channel. The relationship of the received signals to the transmitted signals may be described as follows:yk=HkT[sk]+nk, k=0, 1, . . . , Nf−1  (Eq. 1)where the index k identifies the subcarrier and Nf is the number of subcarriers; yk is a complex vector of NR components corresponding to the signals received at each of the NR receive antennas; sk is the symbol vector representing the source data stream; Hk is a NR×NT matrix whose components represent the complex gain of the channel; and nk is a vector representing the noise received at each receiving antenna. T[sk] represents the transmitter spatial processing that maps the symbol vector sk onto the NT transmit antennas. (In the discussions herein, the following notational conventions are used, unless otherwise specified: bold capital letters represent matrices; bold lowercase letters represent vectors; and italicized letters represent scalar quantities.)
The characteristics of the propagation paths typically vary over time due to a number of factors such as, for example, fading, multipath, and external interference. Consequently, the transmitted RF modulated signals may experience different channel conditions (e.g., different fading and multipath effects) and may be associated with different complex gains and signal-to-noise ratios (SNRs). In equation (1), these characteristics are encoded in channel response matrix Hk.
To achieve high performance, it is often necessary to characterize the response Hk of the wireless channel. The response of the channel may be described by parameters such as spectral noise, signal-to-noise ratio, bit rate, or other performance parameters. The transmitter may need to know the channel response, for example, in order to perform spatial processing for data transmission to the receiver as described below. Similarly, the receiver may need to know the channel response to perform spatial processing on the received signals to recover the transmitted data.